Prior ArtThe following is a tabulation of some prior art that presently appears relevant:U.S. Pat.Pat. No.Kind Code Issue DatePatentee5,554,392Sep. 10, 1996 Gray4,787,597Nov. 29, 1988 Yokota et al3,197,964Aug. 3, 1965 Fehlmann et al2,791,819May 14, 1957 Carlsen2,253,730Aug. 26, 1941 Seailles2,096,159Oct. 19, 1937 Brynoldt1,647,685Aug. 25, 1925CoopersU.S. patent applicationsapplication No.Filing DateApplicant13/373,816Dec. 1, 2011Kreizinger
This invention discloses a method of using the thixotropic properties of no-slump concrete to significantly reduce the hydrostatic pressure that freshly mixed concrete exerts on a vertical form. Thixotropy is a material property that describes the ability of a highly thixotropic material, such as no-slump concrete, to change from a semi-solid state or gel-like state when at rest to a liquid state when vibrated. While in its semi-solid state, freshly mixed no-slump concrete exerts little or no hydrostatic pressure on the forms into which it is cast. Reducing the hydrostatic pressure in forms by minimizing the amount of concrete in a liquid state enables relatively weak materials to be used as forms and reduces the amount of bracing.
Cast-in-place concrete construction using forms to contain the freshly mixed concrete is the most widely used method of building vertical concrete structures such as walls and columns. Cast-in-place means that a concrete structure is cast in its final installed place and will not be moved as opposed to precast or tilt-up concrete that is cast in one place and installed in a second place. The process is based upon casting freshly mixed concrete into forms that are erected to several feet in height and are referred to herein as a “vertical form”. In order to get the concrete to flow to the bottom of the vertical form and fill the entire form side to side, a higher slump and more liquid concrete is typically used. Once cast inside the form the concrete is vibrated for consolidation (the removal of entrapped air) and to ensure the concrete fills the entire form from side to side. Vibration is not needed when the concrete mix is a self-consolidating concrete that uses additives to produce a highly liquid concrete that acts similar to water in removing the air and filling the form.
It is well known in the art that concrete requires a relatively small amount of water to enable the hydration process to fully cure the concrete to its highest strength. However, this small amount of water produces a very dry, semi-solid state-like concrete that is unworkable in most applications and even more so when filling a tall and narrow form. To improve the concrete's workability, additional water and/or chemical additives are added that alters the concrete into more of a liquid state which easily flows into a vertical form. The degree of a concrete mixes' liquidity is typically revealed by the slump test with a very low or no-slump indicating a concrete mix in a semi-solid state and a very high slump indicating a highly liquid mix. As such, freshly mixed concrete can have the characteristics of a solid or a liquid.
One characteristic of a liquid is the existence of hydrostatic pressure which creates a major obstacle in concrete formwork since a liquid concrete weights about 150 lbs. per cubic foot which results in high lateral pressures on a vertical form. For example when a more liquid concrete is cast into a ten foot tall by ten foot long vertical form, the hydrostatic pressure along the bottom can be as high as 1,500 lbs. per square foot and the entire 100 square foot form can have as much as 53,000 lbs. of hydrostatic pressure that must be safely restrained. In order to handle such high amounts of pressure the concrete forms must be very strong, durable and well braced which makes them expensive. This is the reason concrete formwork accounts for as much as 60% of the cost of a plain cast-in-place concrete wall that many times has unsightly exposed form tie holes or patches.
In addition to the hydrostatic pressure caused by the liquefied concrete in the form, in some instances there may also be a vibratory pressure caused by the active vibration of the concrete that must be considered. While the hydrostatic pressure may be present throughout the entire form, the vibratory pressure is localized to the immediate area where the concrete is being vibrated. When combined the hydrostatic and vibratory pressures may magnify the lateral pressure exerted on the form and thereby require even stronger and more expensive forms.
The hydrostatic pressure has been restrained in concrete forms by using a combination of strong form materials, braces, studs, walers, form ties and clamps that support or hold the form sides together and are all well known in the art. Regardless of the forming system used, there is a direct relationship to the forming system's cost and the amount of hydrostatic pressure that must be safely restrained. The greater the hydrostatic pressure, the greater the cost of the forming system and a substantial reduction in the hydrostatic pressure will cause a substantial reduction in the cost of concrete formwork.
The existence of high hydrostatic pressure also limits the type of form material that may be safely or practically used and thereby prevents the use of finished cladding materials as stay-in-place forms. Finished claddings such as siding boards and brick and stone panels are not designed to withstand high lateral pressures, leaks or to be used with form ties and are therefore only attached to a hardened concrete wall. The result is redundant steps of setting and removing heavy concrete forms and then attaching the finished cladding as opposed to simply setting the finished cladding as stay-in-place forms. The additional steps of setting and removing formwork add considerable cost to the construction process.
The utilization of stay-in-place forms is well understood such as insulated concrete forms that provide both formwork and the building's insulation. However, since these insulated concrete forms are used with a more liquid, higher slump concrete, they are specially fabricated and require numerous form ties in very close proximity which increase their material and labor costs and thereby make them only slightly more cost effective than using removable forms.
The high levels of hydrostatic pressure also make it difficult, and thereby expensive, to build walls with architectural cast-in-place concrete. The form ties, which are typically used in cast-in-place construction to hold down costs, inhibit the use of form liners due to the fact that form ties obstruct the form faces and the cavity area between two sides of forms. Form ties are internal bracing that hold two sides of a form together by either connecting to or penetrating through the form faces and prevents unobstructed form faces which are best when using form liners or a stay-in-place material. The alternative of not using form ties require that the forms be extremely strong and able to transfer the pressure loads to the form's perimeter which results in even more costly and economically unfeasible forms.
Another problem with a more liquid concrete is that it requires that the form seams be much tighter and in the same plain so as to prevent leakage or an unsightly ridge on the hardened concrete. In addition, a more liquid concrete mix more readily flows into all openings and thereby inhibits the ability to use slip form stone masonry to build inexpensive brick or stone walls. Slip form stone masonry is the stacking the bricks or stones on the inside of a form and casting concrete behind them to build a brick or stone veneer concrete wall. This is only practical if the concrete is prevented from leaking to the front and staining the brick or stone which is almost impossible when using a highly liquefied concrete.
Despite the limitations caused by and the cost of dealing with hydrostatic pressure, there is no prior art that reduces the hydrostatic pressure in cast-in-place concrete except the standard practice of using a slower casting rate. When the concrete is cast and vibrated at a slower rate it gives the concrete at the bottom of the form time to setup (solidify) and thereby withstand the above hydrostatic pressures. However, a 50% slower casting rate may reduce only 30% of the hydrostatic pressure in the forms while taking twice as long to cast the concrete. At best a slower casting rate process only reduces a relatively small amount of pressure in the forms and there are many other variables that affect the speed in which the concrete begins to set up that limit the effectiveness of this practice.
Another way of reducing the hydrostatic pressure in cast-in-place concrete forms is by using a lighter weighing concrete. There are certain lighter weighing aggregates that can reduce the concrete weight by about 20% although they cost more and are only found in certain areas of the country which make their use cost prohibitive in most of the country. Foam or air injected into the concrete can also lighten it but the resulting concrete is much weaker, costs more and is seldom used.
Pneumatically spraying concrete is the only existing placement method that virtually eliminates the existence of hydrostatic pressure in freshly cast concrete. However, this process is not that widespread in buildings due to the additional placement and finishing labor and the high levels of rebound waste that combine to make it cost about the same as a formed concrete wall although with a somewhat lower quality finish.
There is no prior art that discloses a method of reducing the concrete's hydrostatic pressure in forms through the utilization of the thixotropic properties of the freshly mixed concrete. The thixotropy of freshly mixed concrete changing from a solid state to a liquid state and back to a solid state was disclosed in U.S. Pat. No. 2,253,730, although the method disclosed was for the rapid demolding of cast concrete products. The present invention uses this same material property for a very different purpose—to substantially reduce the hydrostatic pressure created by casting freshly mixed concrete into a vertical form.